The invention relates to an FM receiver of the type that includes means for controlling the center frequency of an IF filter, which is relatively narrow-banded with respect to the channel bandwidth, in dependence on the desired modulation, the receiver also including a signal processing unit with an IF unit and an LF unit for amplifying, frequency converting, and demodulating the received FM signal.
Circuits of this type are disclosed in DE-A 3,147,493 and DE-A 3,438,286, EP-A 0,075,071 or FR-A 8,121,986, all originating from Jens Hansen as the inventor.
The performance capability of conventional VHF receivers is hardly able to handle present-day difficult receiving conditions. The dense occupation with stations creates a "spectral sump" which clearly shows the limits of receiver design regarding receiving sensitivity and selectivity. The dilemma of VHF reception is that the spectrum of a frequency modulated signal has no defined limits but runs out slowly and thus, inevitably, in spite of all limitation measures at the transmitter, projects into the adjacent channels. Even a medium field intensity stereo transmitter noticeably reduces the threshold sensitivity of its adjacent stations. If now, as in mobile operation, receiving conditions change constantly, increased noise, takeover of modulation, and reflection distortions are the unavoidable result.
Conventional filter technology cannot help here because the spectral lines of noise within the receiving channel can no longer be selected. Although contraction of the band-width to reduce multipath interferences would bring part of the noise spectrum outside of the filter, it would also bring part of the useful spectrum outside of the filter, so that the receiving sensitivity would be reduced.
In the known methods for processing FM signals in the IF domain described in the above-mentioned references, controllable, narrowband filters are employed. These narrowband filters, whose resonance position is controllable, perform at the same rhythm as the IF signal to be selected; thus they follow the momentary IF so that a selection is made at every moment precisely where a selection happens to be required. Due to the narrowbandness of the employed filters, sensitivity and selectivity are increased considerably.
The control voltage for the resonant frequency of the filters is derived from the low frequency, whose level is a sufficient measure for the position of the momentary IF. However, for reasons of principle, this position indication cannot be accurate since, in order to control the filters, the IF must first pass through them and must then be demodulated. Thus, any control always takes place with a certain delay which is essentially given by the group delay of the filters. The more narrowband the filters are designed, the higher is their group delay. In simple tracking filter methods or in the known PLL or synchronous methods, this follow-up delay would have the result, in the higher low-frequency range, that the movement of the IF signal and that of the filter would go in opposite directions. Pulling of the filters in the lower LF range would be opposed by pushing in the upper LF range so that ultimately broadband transmission would be impossible.